Analytical chemistry is concerned with both identification and quantification of chemicals. Trace and ultratrace analysis is focused on miniscule quantities. Trace and ultratrace analysis can be important in many areas of analytical chemistry. For example, it can be useful in forensic analysis, quality control, and the analysis of toxins. Sample preparation, including preconcentration, is a key step for almost all analyses. Sample preparation and preconcentration, along with highly sophisticated instrumentation becomes increasingly important when trace and ultratrace detection of analytes is necessary. In addition to more historic sample preparation techniques such as liquid-liquid extraction and simple solvent evaporation, multiple techniques have been introduced over the past several decades to aid analysts in determined low concentrations of analytes such as solid-phase extraction (SPE), solid-phase microextraction (SPME), and stir bar sorptive extraction (SBSE). While the compatibility of each analyte with these techniques must be individually evaluated, these sample preparation techniques have generally produced decreased limits of detection, decreased analysis times, and decreased organic solvent use. Although current sample preparation techniques are excellent for analysis in the parts-per-billion (ppb; μg/L) and high parts-per-trillion (ppt; ng/L) range, ultratrace analysis is still very difficult for analytical chemists to achieve without arduous sample preparation and/or extremely sophisticated and expensive instrumentation. In some cases, even with the most sensitive instruments, analysis of a compound of interest at a desired concentration may still not be possible without concentrating large sample volumes (e.g., greater than 500 mL). Therefore a technique with the ability to more easily determine ultratrace concentrations of interest could be a foundational technique in the field of analytical sample preparation. Furthermore, such a technique would allow more applicable standards to be set of toxic compounds of interest and help ensure human health, to include safe drinking water standards.
The ability to analyze contaminants at ultratrace concentrations is a critically important, but currently challenging, aspect of ensuring safe drinking water. The EPA Maximum Contaminant Level Goal (MCLG) is zero for several compounds, but the enforceable contaminant level, called the Maximum Contaminant Level (MCL), is set, in part, according to the ability of laboratories to measure accurately and consistently the level of the contaminant with available analytical methods. This typically limits MCLs to the parts-per-billion or microgram per liter range. The ability to more easily analyze compounds at extremely low concentrations would be a tremendous achievement in the analytical chemistry field and would have a broad impact, including, for example, in assisting with the provision of safe drinking water. The present invention allows analysis of select compounds in liquid solution at much lower limits-of-detection (LODs) than currently available. In the absence of the present invention, performing trace and ultratrace analysis of certain classes of solutes in drinking water samples, and ultimately ensuring safe drinking water, will remain difficult and force undesirable analytical alternatives (e.g., expensive instrumentation and/or concentrating large sample volumes).
In the past several decades, multiple sample preparation techniques have been introduced to extend the reach of analytical methods into the low parts-per-billion, parts-per-trillion, and even parts-per-quadrillion range by purifying and pre-concentrating target analytes and using sophisticated instrumentation. Sample preparation techniques such as SPE, SPME, and SBSE can extend the concentration range for analysis to ultratrace levels. Although these sample preparation techniques may work well for certain analytes, each has disadvantages, such as limited preconcentration factors among others. For example, SBSE has limitations on the type of analytes for which the procedure can be used due to the limited number of sorbent phases available. Further, SPE typically has a need for a drying step prior to analysis.
Further, although freeze concentration (FC) has received little attention the field of analytical chemistry, it has been used for decades to concentrate solutes from solutions and has found common application in the food industry for creating frozen concentrates of fruit juices and ice beer. As the name implies, FC is a technique for concentrating solutes by freezing a solution. Solutes are concentrated based on the direct relationship between freezing point depression and solute molality. If a solution is slowly frozen, local regions of solvent with a low solute concentration are frozen first and the solution left behind is more concentrated. Although higher solute concentrations decrease the freezing point of a solution, the rate of freezing is generally too fast to freeze out pure solvent when high solute concentrations are present. Therefore, the solution is typically stirred vigorously during FC to counteract incorporation of the solute into the frozen solvent. However, FC is indiscriminant as to the solutes that are concentrated.
Accordingly, it is an objective of the claimed invention to develop a method and apparatus of selective preconcentration and isolation which can discriminate particular compounds. The method and apparatus may be used to facilitate trace and ultratrace analysis or isolation of high value compounds.
A further object of the invention is to allow the trace and ultratrace analysis of toxic chemicals which currently cannot be easily analyzed at the desired concentrations (e.g., maximum contaminant levels; MCLs).